A microbolometer is an elemental infrared detector that utilizes a material whose electrical resistance changes as a function of temperature. This characteristic is used to measure amounts of infrared radiation incident on the microbolometer. When used as an infrared detector, a microbolometer is generally thermally isolated from its supporting substrate or surroundings to allow the absorbed incident infrared radiation to generate a temperature change in the bolometer material, and be less affected by substrate temperature. Example microbolometers are described in U.S. Pat. Nos. 5,021,663; 5,286,976; 5,300,915; 5,420,419; 5,756,999; 5,811,808; and 6,023,061, each of which is hereby incorporated by reference.
Microbolometer detector arrays are used to sense incident infrared radiation at a focal (or image) plane. Each microbolometer detector element in the array absorbs some of the infrared radiation resulting in a corresponding change in the pixel temperature, which results in a corresponding change in resistance. With each microbolometer detector element functioning as a pixel, a two-dimensional image or picture representation of the incident infrared radiation is generated by translating the changes in resistance of each microbolometer detector element into a time-multiplexed electrical signal that can be displayed on a monitor or stored in a computer.
The circuitry used to perform this translation can be called detector array interface circuitry (DAIC) and typically fabricated as an integrated circuit on a silicon substrate. The microbolometer can be fabricated on top of the DAIC. The combination of the DAIC and microbolometer array is commonly known as a microbolometer infrared Focal Plane Array (FPA), or simply MFPA for short. MFPAs can contain, for example, 640×480 microbolometer detector elements.
Certain types of thermal cameras employ MFPAs as the image sensor. However, the microbolometer detector elements of the MFPA will have non-uniform responses to uniform incident infrared radiation, even when the bolometers are manufactured as part of a MFPA. This is due to small variations in the detectors elements' electrical and thermal properties as a result of the manufacturing process. These non-uniformities in the microbolometer response characteristics must be corrected to produce an electrical signal with adequate signal-to-noise ratio for image processing and display as part of the thermal camera operation.
Methods for implementing a DAIC for microbolometer arrays have used an architecture wherein the resistance of each microbolometer is sensed by applying a uniform electric signal source (bias), e.g., voltage or current sources, and a resistive load to the microbolometer detector element. The current resulting from the applied voltage (bias) is integrated over time by an amplifier to produce an output voltage level proportional to the value of the integrated current. The output voltage is then multiplexed to the signal acquisition system.
Gain and offset corrections are applied to the output signal to correct for the non-uniformity. This process is commonly referred to as two-point correction. In this technique, two correction coefficients are applied to the sampled signal of each microbolometer detector element. The gain correction is implemented by multiplying the output voltage by a unique gain coefficient. Likewise, the offset correction is implemented by adding a unique offset coefficient to the output voltage.
Blind (reference) non-uniformity corrections (NUCs) are currently generated in an FPA calibration process by the following process: 1) start with default NUCs stored in the camera's memory, (e.g., flash memory); 2) iteratively capture an image; 3) upload the captured image to external computing resources, such as a personal computer for calculation; 4) generate a set of refined NUCs; 5) download the refined NUC data back into the thermal camera; and 6) write the corrections to flash memory in the thermal camera.
For a specific temperature point in the temperature range at which a MFPA is calibrated, the prior art method of capture/upload/calculation/download/flash-write may be performed up to eighty times. This approach requires considerable time and related expense. Almost half of that calibration time is spent on pixel characteristic/response data collection and processing. The other half is spent on environmental temperature chamber settling time necessary for each new temperature set-point. The calibration processing system typically comprise a communication link between the thermal camera and an external calibration computer running a specialized software suite, such as a LabView® application. This MFPA calibration process is a relatively long and tedious task, with the various steps consuming up to ten hours for a single MFPA over at temperature range from −20° C. to 52° C. in 4° C. increments.